This invention relates to papermaking, and specifically to a papermaking belt, such as a shoe press belt to be used on an open type shoe press paper machine, or a sheet transfer belt.
In an open type shoe press, a xe2x80x9cshoe press beltxe2x80x9d passes around a plurality of rolls and runs through a nip between a press roll and a shoe. A wet paper sheet, sandwiched between felt belts, moves through the nip with the shoe press belt, and is compressed between the press roll and the shoe to squeeze out water.
For high speed operation of a paper machine, a transfer belt is used. A wet paper sheet produced on a forming wire belt is separated from the forming wire belt by a felt pickup belt wound around a pickup roll provided with suction glands. The wet paper sheet, adhering to the outer surface of the felt pickup belt, is conveyed to a press nip formed by upper and lower press rolls, between the felt pickup belt and a sheet transfer belt. When the wet paper sheet is compressed in the press nip, water is transferred from the wet paper sheet to the felt pickup belt. After passing through the press nip, the felt pickup belt is separated from the wet paper sheet.
The wet paper sheet is then conveyed further by the sheet transfer belt to a second press nip. The sheet transfer belt has a flat, smooth, water-impermeable surface, preventing rewetting of the paper sheet, which would occurs if a felt belt were used.
The wet paper sheet is squeezed again at the second press nip between the sheet transfer belt and another felt press belt. This nip may be formed either by two press rolls or by a press roll and a shoe in combination with a press belt. The sheet transfer belt is separated from a wet paper sheet by a guide roll. The wet paper sheet is carried by the sheet transfer belt or by a felt belt, which prevent the wet paper sheet from breaking and make it possible to form the wet paper sheet at a high speed.
In the open type shoe press paper machine, a lubricating oil is sprayed by an oil spraying device onto the inner surface of the shoe press belt at a position immediately ahead of the shoe to reduce friction between the inner surface of the shoe press belt and the shoe. The lubricating oil is scraped off the belt by a scraper and an oil removing brush, both disposed beyond the shoe.
Most papermaking belts heretofore used on open type shoe press paper machines have a base layer and a resin layer coating the surface on the shoe-facing side of the belt. A belt of this type will be referred to as xe2x80x9cone-surface coated beltxe2x80x9d. Belts recently introduced into the market have a base layer, a resin layer formed on the inner surface of the base layer, and a thin resin layer formed on the outer surface, with a view to enhancing abrasion resistance and draining performance. A belt of this type will be referred to as xe2x80x9coutside-covered one-surface coated beltxe2x80x9d. Papermaking belts for achieving closed draw recently introduced into the market have a construction opposite to that of the outside-covered one-surface coated belt. A belt of this type will be referred to as xe2x80x9cinside-covered, one-surface coated beltxe2x80x9d.
The edges of the one-surfaced coated belt, the outside-covered, one-surface coated belt and the inside-covered, one-surface coated belt have a tendency to curl toward the resin layer side while the belt is in use. The transfer belt also has a tendency to curl toward the resin layer side. This tendency to curl is caused by the greater shrinkage of the resin layer relative to the shrinkage of the adjoining base layer. The shrinkage of a resin layer formed by a hot melt coating process is even greater than that of a resin layer formed by a liquid resin application.
Excessive curling of the belts causes gaps to form between the oil scraper and the inner surface of the belt, reducing the effectiveness of the scraper in removing lubricating oil from the belt. If the lubricating oil is not scraped off satisfactorily, the oil remaining on the belt is transferred to a roll and scattered as an oil mist. Consequently, the consumption of the lubricating oil increases, costs increase, the environment around the paper machine is soiled by lubricating oil and waste water will also contain lubricating oil.
Further problems caused by curling of the belt are that the contact of the side edges of the belt with guide palms becomes unstable, and the side edge of the belt tend to catch on ends of the rolls in the process of installing the belt onto the rolls.
One-surface coated belts are disclosed in Japanese Patent Publications Nos. 38477/1988, 15398/1988 and 64639/1991 and Japanese Unexamined Patent Publication Nos. 82988/1992 and 311591/1993, but no mention is made of the curling of the side edges of the belts.
The general object of this invention is to solve one or more of the aforementioned problems. It is also an object of the invention to provide a papermaking belt for use as an open type shoe press belt of a one-surface coated type or an outside covered, one-surfaced coated type or a sheet transfer belt of a one-surface coated type or an inside-coated, one-surfaced coated type, comprising a base layer and a resin layer, and capable of reducing or preventing the curling of side edges thereof due to the difference in thermal shrinkage between the base layer and the resin layer.
With the foregoing objects in view, according to a first aspect of the invention, a papermaking belt of a one-surfaced coated type for use as an open type shoe press belt or a sheet transfer belt comprises a base layer and a resin layer formed on the outer or the inner surface of the base layer when the papermaking belt is mounted on a paper machine, and the resin layer is formed so that the thickness of opposite side edge parts thereof is smaller than that of a middle part thereof to suppress the differential shrinkage effect intrinsic to one-surface coated belts.
According to a second aspect of the invention, a papermaking belt of an outside-covered one-surface coated type or an inside-covered one-surface coated type comprises a base layer, a thin resin layer formed on one of the surfaces of the base layer when the papermaking belt is mounted on a paper machine, and a thick resin layer formed on the other surface of the base layer. The thickness of the thick resin layer decreases widthwise from a middle part of the thick resin layer toward the side edges of the same to suppress the differential contraction effect which is also intrinsic to outside-covered one-surface coated belts and inside-covered one-surface coated belts.